Fire detectors have been widely installed in both commercial buildings and residential structures, such as homes and apartments, to protect the inhabitants and/or other contents located within these structures. These fire detectors are generally of one of the following three types: flame detector; thermal detector; or smoke detector. These three classes of detectors correspond to the three primary properties of a fire: flame, heat and smoke.
Flame Detectors: A flame detector responds to the optical energy radiated from a fire and typically responds to the nonvisible wavelengths. A first class of these detectors operates in the ultraviolet (UV) region below 4,000 .ANG. and a second class, of these detectors operates in the infrared region above 7,000 .ANG.. To prevent false alarms from other sources of UV or infrared light, these detectors are constructed to respond only to radiation in one of these two regions which varies in intensity at a frequency characteristic of typical flicker frequencies of flames (i.e., at a frequency in the range from 5 to 30 Hertz).
Although flame detectors exhibit a low rate of false alarms, they are relatively complex and expensive. Thus, these detectors are generally used only for applications in which cost is not a significant factor. For example, this type of detector is commonly used in industrial environments, such as in aircraft hangers and nuclear reactor control rooms.
Thermal Detectors: Heat from a fire is dissipated by both laminar and turbulent, convective flow. The convective flow is produced by the rising, hot air and combustion gases within the plume of the fire. The two basic types of thermal detectors are: ones that detect when a threshold temperature has been exceeded; and ones that detect when a threshold rate of temperature increase has been exceeded. Temperature threshold detectors are reliable, stable and easy to maintain, but are relatively insensitive. This type of detector is rarely used, especially in buildings having high air flow ventilation and air conditioning systems.
Rate-of-rise thermal detectors are typically used only in environments in which any fires will be expected to be fast-burning fires, such as chemical fires. The threshold for these detectors is typically on the order of 15 Fahrenheit degrees per minute. Unfortunately, there is a significant rate of false detections for both of these two types of thermal detectors.
Recently, a third class of thermal detectors has been introduced that indicates the presence of a fire only if both the temperature and rate of rise of the temperature exceed their respective thresholds. Although this eliminates a high fraction of the false detections, it also makes these detectors highly susceptible to failing to detect the actual occurrence of a fire. This requires that the location of these detectors be carefully selected. Because of this, this type of fire detector is seldomly used in residences.
Smoke Detectors: By far, the most widely-used type of fire detector is the smoke detector. These detectors typically respond to both visible and invisible products of combustion. The visible products typically consist of carbon and carbon-rich particles produced by a fire. The invisible products typically have a diameter of less than 5 microns. The two classes of smoke detectors are: photoelectric detectors that respond to visible products of combustion; and ionization type detectors that respond to both visible and invisible combustion products.
For the past two decades, the ionization type smoke detectors have dominated the fire detector market, because they are less complicated and expensive than flame detectors and thermal detectors. In addition, the ionization type detectors can operate for a year on just a single 9-volt battery available in any super market. In a first class of these devices, the ionization is produced by in the region between a pair of electrodes across which a voltage is produced sufficient to ionize gas in that region. In a second class of these devices, the ionization is produced by generation of a high-speed ion, such as an alpha particle through radioactive decay, which is directed through a sample of air within the room to ionize this sample.
Unfortunately, although the low cost of this second class of ionization type smoke detectors has led to their use in over 90% of households, few people would use these detectors if they were not mandated by fire codes, because of their high rate of false alarms. Few things in life are more irritating than having to dash out of a morning shower to turn off a smoke alarm that has been triggered by steam from a hot shower. These detectors are also easily triggered by smoke produced within a kitchen during meal preparation or even by over-zealous dusting. Because of this, a large fraction of such fire detectors are disabled part or all of the time. The problem of false alarms is thus not only irritating, it is dangerous because of the inclination to disable such detectors to avoid these false alarms.
In one class of smoke detectors designed to reduce the rate of occurrence of such false alarms, a heat detector module is also included in such fire detector and an alarm is produced only if the detection thresholds for both the smoke and heat detectors are exceeded. In another analogous hybrid smoke detector that is similarly designed to reduce the rate of occurrence of these false alarms, a flame detector module is also included and an alarm is produced only if the detection thresholds for both the smoke and flame detectors are exceeded.
Although these two hybrid devices do indeed exhibit a reduced rate of false alarms, each does so in a dangerous manner. First, by producing an alarm only when both the smoke and heat detector modules detect the occurrence of a fire or when both the smoke and flame detector modules detect the occurrence of a fire, this roughly doubles the rate of failure of detecting an actual fire. More precisely, the rate of failure of detecting actual fires is equal to the sum of the rates at which either fails to detect an actual fire, minus the rate at which both would concurrently fail to detect such actual fire. This failure rate is therefore almost equal to the sum of the rates of failure of each of these detector modules individually.
Second, even in those cases in which this fire detector successfully detects the occurrence of an actual fire, the alarm is produced only at such time that both detector modules have detected the occurrence of a fire. Therefore, these hybrid detectors are each slower to respond than either of its detector modules separately. Thus, again, the benefit of a reduced rate of false alarms is achieved at the cost of reducing performance substantially to the lower of the performance levels of its two types of fire detector modules.
A second problem with the ionization type smoke detectors is the relatively slow speed of detecting a fire. Although the speed can be increased by lowering the detection threshold, this increases the rate of false detection and therefore increases the likelihood that it will be intentionally disabled.
A third problem is the need to locate these detectors carefully to achieve a high rate of detection of fires in a household environment. Because smoke is a complex, sooty molecular cluster that consists mostly of carbon, it is much heavier than air and therefore diffuses relatively slowly. This requires that such detectors be located near likely sources of fire in the household environment so that a fire will be detected promptly.
A fourth problem is that, although smoke usually accompanies a fire, the amount of smoke that is produced varies over a wide range depending on the composition of the material that catches fire. For example, certain plastics, such as polymethylmethacrylate, an oxygenated fuels, such as ethyl alcohol and acetone, produce substantially less smoke than hydrocarbon polymers, such as polyethylene and polystyrene. Indeed, some fuels, such as carbon monoxide, formaldehyde, metaldehyde, formic acid and methyl alcohol burn with nonluminous flames and without producing any smoke.
A more indirect problem with ionization detectors is that they typically utilize a radioactive source, such as Americium, as the source of the ionization-producing radiation. Although the amount of such radiative material in any single detector is very small (typically on the order of tens of milligrams), the half-life of Americium and cobalt-60 (two typical radioactive sources) is each over 1,000 years so that, as more and more of these detectors are dumped into our land fills, the more that this can be a problem to future development of these land fills. This can therefore become a problem when tens of millions of these are disposed of every year.
One additional disadvantage of these ionization detectors is that the need for a battery introduces an ongoing cost of maintenance, but more seriously introduces the likelihood that such detectors can become inoperative, because this battery goes dead without such event being realized by the tenant.
An alternate line of fire detectors are based on measurements of the concentration of carbon dioxide. The following three U.S. patents also include circuitry to avoid or at least reduce the occurrence of false alarms. In U.S. Pat. No. 5,053,754 by Jacob Y. Wong entitled Simple Fire Detector, 4.26 .mu. light is directed through a sample of room air to measure the concentration of carbon dioxide in this air, because carbon dioxide has a strong absorption peak at this wavelength. Both the concentration and the rate of change of concentration of the carbon dioxide are measured, enabling an alarm to be generated whenever either of these measured values exceeds a respective threshold value. Preferably, an alarm is sounded only if both of these values exceeds its respective threshold value.
In U.S. Pat. No. 5,079,422 by Jacob Y. Wong entitled Fire Detection System using Spatially Cooperative Multi-Sensor input Technique a set of N sensors are spaced throughout a large room or unpartitioned building. Comparison of data from different sensors provides information that is unavailable from only a single sensor. The data from each of these sensors and/or the rate of change of such data is used to determine whether a fire has occurred. The use of data from more than one sensor reduces the likelihood of a false alarm.
In U.S. Pat. No. 5,103,096 by Jacob Y. Wong entitled Rapid Fire Detector, a black body source produces light that is directed through a filter that transmits light in two narrow bands at the 4.26 micron absorption band of carbon dioxide and at 2.20 microns at which none of the atmospheric gases has an absorption band. A blackbody source is alternated between two fixed temperatures to produce light directed through ambient gas and through a filter that passes only these two wavelengths of light. In order to avoid false alarms, an alarm is generated only when both the magnitude of the ratio of the measured intensities of these two wavelengths of light and the rate of change of this ratio are both exceeded.